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Management of Musty Off-Flavor in Channel Catfishfrom Commercial Ponds with Weekly Applications ofCopper SulfateKevin K. Schrader a , Craig S. Tucker b , Terrill R. Hanson c , Patrick D. Gerard d , Susan K.Kingsbury b & Agnes M. Rimando aa U.S. Department of Agriculture, Agricultural Research Service, Natural Products UtilizationResearch Unit, National Center for Natural Products Research, Post Office Box 8048,University, Mississippi, 38677, USAb National Warmwater Aquaculture Center, Mississippi State University, Post Office Box 197,Stoneville, Mississippi, 38776, USAc Department of Agricultural Economics, Mississippi State University, Post Office Box 9755,Mississippi, Mississippi State, 39762, USAd Experimental Statistics Unit, Mississippi State University, Post Office Box 9653, Mississippi,Mississippi State, 39762, USA

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To cite this article: Kevin K. Schrader, Craig S. Tucker, Terrill R. Hanson, Patrick D. Gerard, Susan K. Kingsbury & Agnes M.Rimando (2005): Management of Musty Off-Flavor in Channel Catfish from Commercial Ponds with Weekly Applications ofCopper Sulfate, North American Journal of Aquaculture, 67:2, 138-147

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North American Journal of Aquaculture 67:138–147, 2005 [Article]q Copyright by the American Fisheries Society 2005DOI: 10.1577/A04-051.1

Management of Musty Off-Flavor inChannel Catfish from Commercial Ponds with

Weekly Applications of Copper Sulfate

KEVIN K. SCHRADER*

U.S. Department of Agriculture, Agricultural Research Service,Natural Products Utilization Research Unit,

National Center for Natural Products Research,Post Office Box 8048, University, Mississippi 38677, USA

CRAIG S. TUCKER

National Warmwater Aquaculture Center, Mississippi State University,Post Office Box 197, Stoneville, Mississippi 38776, USA

TERRILL R. HANSON

Department of Agricultural Economics, Mississippi State University,Post Office Box 9755, Mississippi State, Mississippi 39762, USA

PATRICK D. GERARD

Experimental Statistics Unit, Mississippi State University,Post Office Box 9653, Mississippi State, Mississippi 39762, USA

SUSAN K. KINGSBURY

National Warmwater Aquaculture Center, Mississippi State University,Post Office Box 197, Stoneville, Mississippi 38776, USA

AGNES M. RIMANDO

U.S. Department of Agriculture, Agricultural Research Service,Natural Products Utilization Research Unit,

National Center for Natural Products Research,Post Office Box 8048, University, Mississippi 38677, USA

Abstract.—We evaluated the effectiveness of weekly low-dose applications of copper sulfate(0.12 mg of Cu/L of water) for reducing the prevalence of off-flavor in channel catfish Ictaluruspunctatus on commercial farms. The study was conducted over 3 years in ponds (3.2–8.4 ha) ontwo catfish farms in western Mississippi. Farm managers applied copper sulfate (0.5 mg of coppersulfate pentahydrate/L of water) weekly beginning in the late spring or early summer and continueduntil the water temperature dropped below 208C. Water samples were collected from treated anduntreated ponds approximately every 3 weeks during the application period and were monitoredfor levels of the musty compound 2-methylisoborneol (MIB), chlorophyll a, and phytoplanktoncommunity structure and abundance. In addition, channel catfish were caught from each studypond during the third year of the study and were checked for flavor. Levels of MIB and theabundance of the MIB-producing cyanobacterium Oscillatoria perornata were significantly lowerin treated ponds than in control ponds at one farm, while numbers of green algae and diatoms atboth farms were significantly higher in treated ponds than in control ponds. Also, fish flavoranalysis indicated that the overall prevalence of all types of off-flavor was 50% lower in treatedponds than in control ponds. Based upon our results, weekly low-dose applications of copper

* Corresponding author: kchrader@msa-oxford.ars.usda.gov

Received August 13 2004; accepted December 15 2004Published online April 29, 2005

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139MUSTY OFF-FLAVOR MANAGEMENT

sulfate appear to be beneficial in mitigating musty off-flavor problems in commercially producedchannel catfish. Copper sulfate treatment reduced potential harvest delays by nearly half andreduced costs associated with off-flavor by 35%. However, the economic benefit of treatment wasnot statistically significant, although this result is probably attributable to the small data set usedfor economic analyses rather than ineffectiveness of treatment.

‘‘Off-flavor’’ catfish Ictalurus spp. are rejectedby processors and must be held in ponds until fla-vor quality is deemed acceptable. Harvest delaysrelated to off-flavors cause economic losses to pro-ducers by increasing the length of time requiredto raise a crop, creating additional feed costs, andinterrupting cash flow; the extended holding periodalso increases catfish losses from disease, possiblewater quality deterioration, and bird depredation(Tucker 2000). Economic losses to catfish pro-ducers from off-flavor have been estimated to beas high as US$46.7 million annually (Hanson 2001).

The most common off-flavor in farm-raisedchannel catfish I. punctatus in western Mississippiis described as ‘‘musty’’ or ‘‘earthy’’ and is dueto the absorption of 2-methylisoborneol (MIB)into the flesh of the catfish (Martin et al. 1988;van der Ploeg et al. 1992). The cyanobacteriumOscillatoria perornata (also called Planktothrixperornata) produces MIB (Martin et al. 1991) andhas been cited as the most prevalent cause of MIBoff-flavor in western Mississippi catfish ponds (vander Ploeg et al. 1992). One management approachfor controlling musty off-flavor in cultured catfishis the application of algicides such as diuron (3-[3,4-dichlorophenyl]-1,1-dimethylurea; Zimba etal. 2002) or copper-based products such as coppersulfate to kill or prevent the growth of O. perornata(Boyd and Tucker 1998). Although diuron is notregistered for use as an algicide in catfish ponds,it can be used to manage cyanobacteria-related off-flavors in some states under a Federal Insecticide,Fungicide, and Rodenticide Act Section-18 emer-gency exemption that must be renewed each yearby the U.S. Environmental Protection Agency.Copper sulfate, on the other hand, has a long his-tory of use as an algicide in aquaculture and isfully registered for that use.

The main disadvantage to the use of copper-based products as algicides in catfish ponds is thestrong interaction of the cupric ion with other wa-ter quality variables, which makes it difficult toachieve consistently effective results. A poor un-derstanding of copper interactions with other waterquality variables also makes it difficult to safelytreat aquaculture ponds because there is little dif-ference between phytotoxic and ichthyotoxic con-centrations of copper.

An alternative approach to use of the label-recommended rates for copper sulfate as an algi-cide in catfish ponds was initially reported byTucker (1998). Low doses of copper sulfate ap-plied weekly to 0.4-ha catfish ponds were foundto have a positive effect on catfish flavor qualityand to provide direct economic benefit to produc-ers (e.g., increased average net returns above var-iable costs) (Tucker and Hanson 1999; Tucker etal. 2001). Although this treatment protocol wassuccessful under experimental conditions, Tuckeret al. (2001) expressed concern about the potentialefficacy of treatment in the large ponds ($3 ha)used by commercial producers. Specifically, it maybe more difficult to rapidly disperse copper sulfatein large ponds, which may hinder the effectivenessof treatment. The objective of our study was todetermine whether the positive results observed byTucker et al. (2001) were achievable in commer-cial-size catfish ponds. Therefore, we monitoredand analyzed the impacts of weekly low-dose cop-per sulfate applications to commercial-size catfishponds ($3 ha) on certain water quality variables(e.g., MIB levels and phytoplankton communitystructure and abundance), channel catfish flavorquality, and the economics of channel catfish aqua-culture.

Methods

Facilities.—The study was conducted over 3years (2000–2002) during the summer months inearthen, levee-type ponds located at two differentcommercial catfish production operations in Mis-sissippi: one (SC) in Humphreys County, and theother (AC) in Leflore County. Water was suppliedfrom alluvial aquifers, and ponds were managedin the routine manner used by each productionfacility. At the AC location, the same 12 pondswere used during the entire 3-year study period,and both treated and control ponds were 3.2–5.2ha. At the SC location, different ponds were usedduring each summer and autumn as follows: (1)during 2000 and 2001, 16 ponds were used eachyear (treated ponds were 4.1–6.6 ha; control pondswere 4.1–8.4 ha); and (2) during 2002, 12 pondswere used (treated ponds were 4.1–5.3 ha; controlponds were 4.5–6.5 ha).

Catfish aquaculture practices.—The stocking

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rates of the commercial ponds used in this studywere variable and were approximately 65,000–119,000 fish per pond (averaging 26–84 g/fish).Restocking rates were also variable throughout thestudy period and were mainly dependent upon thenumber of channel catfish harvested or lost to dis-eases in the previous year. Channel catfish popu-lations were mixed-size during the course of thestudy, which is typical of the ‘‘multiple-batch’’cropping system used by many Mississippi catfishfarmers (Tucker et al. 1994). During the final yearof the study (2002), channel catfish (two fish perpond) were removed monthly (July, August, andSeptember) from each test pond at both locationsto determine flavor quality.

Copper sulfate treatments.—During the 3-yearperiod for the AC location, six ponds were des-ignated as untreated (control) ponds while six oth-er ponds received weekly applications of 5.6 kgof copper sulfate pentahydrate/ha (0.5 mg coppersulfate pentahydrate/L of water or 0.12 mg Cu/L)during the summer and early autumn months (usu-ally late June or early July to early October). Dur-ing 2002, ponds did not receive weekly applica-tions of copper sulfate for at least 2 weeks priorto sampling on August 28 since the farm ran outof its supply and had to reorder the chemical. Atthe SC location, the number of untreated and treat-ed ponds differed as follows: (1) during 2000,there were 10 untreated ponds and 6 treated ponds;(2) during 2001, there were 8 untreated ponds and8 treated ponds; and (3) during 2002, there were6 untreated ponds and 6 treated ponds. Applica-tions of copper sulfate to ponds at SC were ini-tiated about 1 month earlier (late May or earlyJune) than at AC. For treatment applications, cop-per sulfate crystals (Triangle Brand, Phelps DodgeRefining Corp., El Paso, Texas) were placed in theappropriate quantities in double-layered burlapbags, and these bags were then suspended just be-low the surface of the water column about 5–7 min front of the paddlewheel aerators at the SC lo-cation and 2–3 m behind the paddlewheel aeratorsat the AC location. Aerators were operated for sev-eral hours to dissolve all of the copper sulfate, andthe water current created by the aerators helpeddistribute the copper throughout the pond water.Weekly applications occurred in the morning andwere discontinued each autumn when the watertemperature dropped below 208C.

Water quality analyses.—During 2000, watersamples were obtained from each pond every 2weeks during June and July and then every 3 weeksduring August, September, and early October. Dur-

ing 2001 and 2002, water samples were obtainedfrom each study pond every 3 weeks from late Mayor early June to early October. Water samples werecollected opposite the leeward side of the pondabout 20 cm below the water surface and wereplaced on ice in a cooler for transport to the lab-oratory. Water samples were measured for chlo-rophyll a by chloroform–methanol extraction andspectroscopy (Lloyd and Tucker 1988), and testedfor geosmin and MIB levels (Schrader et al. 2003).For determination of geosmin and MIB levels, wa-ter samples were immediately placed in glass scin-tillation vials with foil-lined caps for transportback to the laboratory. These samples were thenprocessed further by placing 600 mL of water into1.8-mL glass vials containing 0.2 g of NaCl, andanalysis was performed by solid-phase microex-traction and gas chromatography–mass spectrom-etry (Schrader et al. 2003).

Water samples were also processed for micro-scopic examination to determine phytoplanktoncommunity structure and enumeration; 50-mL sub-samples were preserved with Lugol’s solution andstored at 48C. Phytoplankton were then identifiedand counted as ‘‘natural units’’ (colonies, fila-ments, or unialgal cells) in a Sedgewick–Raftercounting chamber at 3003 magnification (APHAet al. 1995). Eukaryotic phytoplankton were iden-tified to the genus level, and cyanobacteria wereidentified to species. The farm manager at the SClocation provided data on total ammonia and nitritelevels in pond water for the 2000 sampling period.

Data analysis.—Data on chlorophyll-a levels,MIB levels, and the abundance of chlorophytes,diatoms, and the most common types of cyano-bacteria observed during this study were combinedfor the 3-year sampling period at each farm lo-cation. Because the data were positively skewed,a log transformation was used before data analysis.Log-transformed yearly totals were analyzed fortreatment and year effects by use of a repeated-measures analysis of variance employing themixed procedure in the Statistical Analysis Sys-tem, version 8 (Littell et al. 1996). Significant ef-fects were further investigated through a compar-ison of least-squares means. A significance levelof 0.10 was used for all hypothesis tests.

Fish flavor testing.—Fish samples were obtainedfrom each study pond once per month during July,August, and September of 2002. Sampling and fla-vor-testing methods were similar to those used bycommercial operations. In our study, two fish werecaptured from each pond, and a trained sensorypanel assessed flavor. Common flavor descriptors

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TABLE 1.—Log-transformed least-squares means ofmeasured water quality variables for control and coppersulfate-treated channel catfish ponds at farm AC (MIB 52-methylisoborneol). The model-based standard errors arein parentheses. Least-squares means within a row that arefollowed by the same letter are not significantly different(P . 0.10).

Variable

Pond treatment

Control Copper sulfate

Chlorophyll a (mg/L) 6.47 (0.08) z 6.17 (0.08) yMIB (ng/L) 6.20 (0.60) z 4.71 (0.60) zOscillatoria perornataa 6.37 (0.70) z 5.48 (0.70) zRaphidiopsis brookiia 9.13 (0.46) z 7.66 (0.46) yO. agardhiia 10.51 (0.16) z 10.00 (0.16) zChlorophytesa 7.47 (0.24) z 8.13 (0.24) yDiatomsa 6.46 (0.18) z 6.93 (0.18) y

a Expressed as natural units/mL.

were used to identify the type of off-flavor (Tuckerand van der Ploeg 1999). The intensity of the off-flavor was rated with a hedonic scale of 0–5, where0 indicated no detectable off-flavor and 5 repre-sented intense off-flavor.

Economic analysis.—The sampling protocol inthis study provided three off-flavor checks persummer period during 2000, 2001, and 2002. How-ever, the economic analysis was restricted to the2002 summer period, when copper sulfate was ap-plied and off-flavor water and fish sampling oc-curred. The economic analysis considered addi-tional production and copper sulfate applicationcosts resulting from off-flavor harvest delays.

Estimation of the number of off-flavor days wasthe basis for quantifying additional operating costsduring the extended harvest delay periods. Hanson(2001) estimated the cost of off-flavor harvest de-lays at $8.85 per hectare per day. This cost estimateincluded additional feeding, labor, sampling, trans-portation, fish losses, and an opportunity cost. Theopportunity cost covered reduced production ef-ficiency caused by harvest delays that negativelyimpacted catfish restocking and growth. FromHanson (2003) and Tucker et al. (2001), the costof applying crystalline copper sulfate was $156per hectare per 155-d summer period (22 appli-cations) or $7.09 per application per hectare; thisinformation was used in estimating copper sulfatetreatment costs. During the July 16–September 18research period, there were eight copper sulfateapplications; total application levels and costswere adjusted for individual pond sizes.

Off-flavor fish sampling results were used to hy-pothetically determine whether fish in a particularpond could be harvested. The first sampling dateof July 16, 2002 (or July 17for the second farmsampled), began the count of ‘‘off-flavor days’’ ifthe pond fish sample was off-flavor, which meantthat no harvest could occur. If a fish sample wasnot off-flavor, then no off-flavor days were countedand the fish were harvestable. The second sam-pling date, August 27, 2002 (or August 28), was42 d after the first sampling date. If fish in a sam-pled pond were again off-flavor, then 42 off-flavordays were assigned to that pond. If the sample wasnot off-flavor, then no off-flavor days were countedfor this period, as fish could be harvested. Thethird sampling date, September 18, 2002 (or Sep-tember 19), was 22 d after the second samplingdate. If fish from a pond were off-flavor on thefinal fish sampling date, then 22 off-flavor dayswere counted for that pond. Various combinationsof on- and off-flavor sampling results per period

occurred for the 24 ponds in this research and af-fected the overall number of harvest delay daysused in the economic analysis.

For estimation of additional off-flavor days afterthe final sampling date (September 18, 2002), re-sults of off-flavor depuration studies were used todevelop a ‘‘best-case’’ assumption for the lengthof time required for MIB to be purged from thefish (i.e., returning fish to an ‘‘on-flavor’’ conditionso that they could be harvested). Johnsen et al.(1996) and Howgate (2003) found that catfish re-quired 3 d to lose the off-flavor from a level-2rating and required 4 d to lose the off-flavor fromrating levels 3–5. This is a conservative approach,as their results were based on the most favorableconditions for depuration (transfer of fish to cleanwater devoid of tainting compounds). In a com-mercial pond setting, depuration times wouldprobably be considerably longer. Resulting harvestdelays due to off-flavor were summed over thethree sampling dates and were used in estimatingperiodic and overall costs.

Costs were further adjusted for pond surface wa-ter area. T-tests (a 5 0.10) were applied to thenumber of off-flavor days at each sampling dateand the total number of off-flavor days for treatedand untreated ponds during the sampling period.Additional production costs related to off-flavorand copper sulfate treatment were also tested fordifferences between treated and untreated pondsby use of t-tests (a 5 0.10).

Results and Discussion

Water Quality

At AC (Table 1) and SC (Table 2), the abundanceof green algae and diatoms (Division Chromophy-

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TABLE 2.—Log-transformed least-squares means ofmeasured water quality variables for control and coppersulfate-treated channel catfish ponds at farm SC (MIB 52-methylisoborneol). The model-based standard errors arein parentheses. Least-squares means within a row that arefollowed by the same letter are not significantly different(P . 0.10).

Variable

Pond treatment

ControlCoppersulfate

Chlorophyll a (mg/L) 6.33 (0.11) z 6.22 (0.13) zMIB (ng/L) 4.78 (0.59) z 2.39 (0.68) yOscillatoria perornataa 3.38 (0.69) z 1.47 (0.79) yRaphidiopsis brookiia 6.96 (0.78) z 5.01 (0.88) zO. agardhiia 7.56 (0.69) z 8.31 (0.79) zChlorophytesa 8.17 (0.20) z 8.76 (0.21) yDiatomsa 6.96 (0.17) z 7.52 (0.20) y

a Expressed as natural units/mL.

ta, Class Bacillariophyceae) was significantlyhigher in ponds receiving weekly low-dose appli-cations of copper sulfate than in control ponds.However, not all of the types of the most commonfilamentous cyanobacteria monitored (Raphidiop-sis brookii, O. perornata, and O. agardhii) had low-er abundance in treated ponds than in controlponds. Only R. brookii in treated ponds at AC andO. perornata in treated ponds at SC exhibited sig-nificantly lower abundance relative to controlponds based upon analysis of log-transformed data(Tables 1, 2). Musty off-flavor in farm-raised chan-nel catfish has not been attributed to the presenceof R. brookii or O. agardhii in catfish aquacultureponds.

Cyanobacteria possess several physiological at-tributes that allow them to dominate phytoplank-ton communities in catfish ponds (Paerl and Tucker1995; Hargreaves 2003). For example, planktoniccyanobacteria can produce gas vacuoles that allowthem to regulate cell buoyancy in the water columnand compete better for light in catfish ponds. Also,cyanobacteria can produce toxins that can inhibitthe growth of other types of phytoplankton(Bagchi et al. 1990; Chauhan et al. 1992). A re-duction in the abundance of filamentous cyano-bacteria at both farms may account for the increasein the numbers of green algae and diatoms.

The chlorophyll-a level (a surrogate measure ofphytoplankton biomass) was significantly lower intreated ponds than in control ponds at AC (Table1). At SC, the MIB level was significantly lowerin treated ponds than in control ponds, a findingthat is corroborated by the lower abundance of O.perornata in treated ponds (Table 2). Geosmin,also produced by certain types of cyanobacteria,

is the other most common off-flavor compoundthat causes an ‘‘earthy’’ taint in catfish; however,it is not prevalent in western Mississippi catfishponds (van der Ploeg et al. 1992). In our study,geosmin was detected infrequently in water sam-ples obtained during the 3-year sampling period.In fact, geosmin was only detected ($10 ng/L) in45 of 564 water samples (8%). The infrequent oc-currence of geosmin in the catfish ponds preventedvalid determination of the efficacy of weekly low-dose applications of copper sulfate for reducinggeosmin level.

The lack of consistency in the effects of coppersulfate treatment on MIB concentration and phy-toplankton abundance at the two farms could bedue to differences in the method or timing of cop-per applications. Farm AC placed the burlap bagscontaining copper sulfate between the aerator andthe banks of the pond instead of in the strong watercurrent in front of the aerator. Farm managementat AC chose this location because the greater ponddepth in front of the aerators made it more difficultfor farm personnel to properly suspend the burlapbags in the water column. The selected locationmay have affected the distribution of the dissolvedcopper sulfate throughout the ponds, thereby re-ducing the effectiveness of the treatments. Also,farm AC did not initiate weekly applications ofcopper sulfate until about 1 month after pond watertemperatures presumably became elevated above208C. At a water temperature above 208C, the spe-cific growth rate of O. perornata is enhanced (vander Ploeg et al. 1995). During the third year of thestudy, delays in applications of copper sulfate atAC may have permitted greater proliferation of O.perornata than occurred in the ponds at SC, whichwere treated earlier. This is evidenced by the high-er mean June abundance of O. perornata in pondsat AC (about 2,000 filaments/mL) than at SC(,200 filaments/mL).

Treatment with copper sulfate will cause dete-rioration in water quality, especially higher totalammonia and nitrite concentrations (data notshown), which was also shown by Tucker et al.(2001). However, one of the benefits of the low-dose protocol used in this study is that deterio-ration of water quality is not as severe as mightbe the case when higher copper sulfate treatmentconcentrations are used.

Off-Flavor in Sampled Channel Catfish

Based upon flavor analysis conducted during2002, ponds receiving weekly low-dose applica-tions of copper sulfate at farms SC and AC ex-

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TABLE 3.—Number of ponds with off-flavored channelcatfish out of six replicate untreated control ponds and sixponds treated weekly with 0.12 mg Cu/L (from coppersulfate pentahydrate) during 2002 (MIB 5 2-methyliso-borneol).

Farm Date

All off-flavors

Control Treated

MIB off-flavors

Control Treated

SC 16 Jul 3 3 2 2SC 27 Aug 5 2 4 1SC 18 Sep 4 2 4 1AC 17 Jul 5 2 4 2AC 28 Aug 4 3 3 2AC 17 Sep 4 1 3 1

TABLE 4.—Rating of 2-methylisoborneol (MIB) off-fla-vors in channel catfish from untreated control ponds andponds treated weekly with 0.12 mg Cu/L (from coppersulfate pentahydrate) during 2002.

FarmaPond

number16–17Julb

27–28Augb

17–18Sepb

Control

SC 9N 0 2 0SC 12N 0 3 5SC 13N 3 2 3SC 19N 0 0 0SC 20N 0 0 3SC 23N 4 3 3

Treated

SC 5S 0 0 0SC 7S 0 0 0SC 16N 4 3 3SC 2W 4 0 0SC 3W 0 0 0SC 4W 0 0 0

Control

AC 113 0 4 5AC 114 3 0 0AC 115 3 0 2AC 160 4 3 0AC 162 0 0 0AC 164 4 2 4

Treated

AC 112 0 0 0AC 116 0 4 3AC 117 0 0 0AC 161 0 0 0AC 163 3 3 0AC 165 2 0 0

a Farm AC missed 2 weeks of application prior to sampling onAugust 28.

b Hedonic scale where 0 5 no detectable MIB off-flavor and 5 5intense MIB off-flavor.

perienced a reduction in the prevalence of MIB-flavored channel catfish by September, and off-flavored channel catfish were present in fewertreated ponds than control ponds (Table 3). Chan-nel catfish rated at level 1 for flavor (van der Ploeg1991) were considered to exhibit a slight off-flavorthat was recognizable but not objectionable. In ad-dition to ‘‘musty,’’ a wide range of off-flavors weredetected in sampled channel catfish, including‘‘woody,’’ ‘‘onion,’’ ‘‘fish oil,’’ ‘‘cardboard,’’‘‘stale,’’ and ‘‘dead fish.’’ None of these off-flavorshave been associated with the presence of certaintypes of cyanobacteria, so treatment with coppersulfate would not necessarily be expected to helpreduce or eliminate their presence in channel catfish.

Off-flavor was detected in 25 of the 36 samplesfrom control ponds (69.4%) and 13 of the 36 sam-ples from treated ponds (36.1%). These results in-dicate about a 50% reduction in overall off-flavorprevalence, which is lower than the 80% reductionreported by Tucker et al. (2001) when copper sul-fate was applied to experimental ponds. For MIBoff-flavor, 20 of 36 samples from control ponds(55.6%) were determined to have a musty flavor,while 9 of 36 samples from treated ponds (25.0%)were musty flavored, indicating a 55% lower MIBoff-flavor prevalence in treated ponds than in con-trol ponds.

At both farms, the rating of MIB off-flavors inchannel catfish also generally showed greater im-provement over time in treated ponds than in con-trol ponds (Table 4). Typically, the prevalence ofmusty off-flavor in commercial pond-raised chan-nel catfish is expected to be highest during the late-summer and early autumn months. At farm SC,only one (pond 16N) of six treated ponds containedMIB off-flavored channel catfish (rating 5 3) nearthe end of the application period, while four of sixuntreated ponds contained MIB off-flavored chan-nel catfish (rating 5 3–5). The channel catfish test-

ed by flavor analysis from pond 16N were larger(approximately 0.9–1.8 kg [2–4 lbs] each) thanany of the other channel catfish (,0.9 kg each)used for flavor analysis from the other ponds atSC. The abundance of O. perornata and MIB levelsin pond 16N did not drop to near zero until thesampling in September (data not shown), so theselarger fish may not have had sufficient time topurge the MIB from their flesh by the time of thesampling.

At farm AC, only one pond (116) of the sixtreated ponds contained MIB off-flavored channelcatfish (rating 5 3) near the end of the applicationperiod, while three of six untreated ponds con-tained MIB off-flavored channel catfish (rating 52–5). Pond 116 yielded on-flavor channel catfishin July of 2002 but was found to contain MIB off-flavored fish during sampling in August and Sep-tember. The lack of weekly applications of copper

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TABLE 5.—Total number of channel catfish off-flavor days calculated for each sampling date in 2002 by treatmentand individual pond. Letters represent significant differences (P , 0.10) for values within a column.

Pond numberor variable

Surfacearea

(hectares)16 Jul–27 Aug

28 Aug–18 Sep After 18 Sepa Total

Control

9N 4.5 0 22 0 2212N 4.5 0 22 4 2613N 4.5 42 22 4 6819N 6.5 0 0 0 020N 6.5 0 0 4 423N 6.5 42 22 4 68

113 5.3 0 20 4 24114 3.6 42 0 0 42115 3.2 42 0 3 45160 4.1 42 20 0 62162 4.1 0 0 0 0164 4.5 42 20 4 66All 57.8 z 252 148 27 427Mean/pond 21.0 z 12.3 z 2.3 z 36 zMean/hectare 4.4 2.6 0.5 7.4

Treated

5S 4.1 0 0 0 07S 4.1 0 0 0 0

16N 4.5 42 22 4 682W 4.1 42 0 0 423W 4.1 0 0 0 04W 5.3 0 0 0 0

112 5.3 0 0 0 0116 3.2 0 20 4 24117 3.2 0 0 0 0161 4.9 0 0 0 0163 3.6 42 20 0 62165 4.5 42 0 0 42All 50.9 z 168 62 8 238Mean/pond 14.0 z 5.2 y 0.7 y 20 zMean/hectare 3.3 1.2 0.2 4.7

a Off-flavor days for the post-18 Sep period were enumerated based on research (Johnsen et al.1996; Howgate 2003) on depuration times required to get catfish on-flavor for various levels of2-methylisoborneol.

sulfate preceding the sampling on August 28,2002, reduced the likelihood of preventing O. per-ornata growth and the resultant MIB off-flavor. Ingeneral, results from the flavor analysis conductedin 2002 indicate that weekly low-dose applicationsof copper sulfate to commercial ponds providesome reduction (55%) in the prevalence of MIBoff-flavored channel catfish; however, the man-agement approach at the commercial level resultedin reductions that were less dramatic than thoseobserved by Tucker et al. (2001).

Economics

The number of off-flavor days for the controland treated ponds (Table 5) were used to calculateadditional off-flavor costs. Even though the controlponds had a total of 427 off-flavor days versus 238off-flavor days for the treated ponds, the averagenumber of off-flavor days was not significantly

different over the entire sampling period becauseof the large variation among ponds and the smallsample size. Individual sampling periods were alsotested for significance; for the period between July16 (July 17 for the second farm sampled) and Au-gust 27 (August 28), there was no statistical dif-ference between the control and treated ponds.However, the number of off-flavor days differedsignificantly for the August 28–September 18 pe-riod: treated ponds had 62 off-flavor days (aver-aging 5.2 off-flavor days per pond), whereas con-trol ponds had 148 off-flavor days (averaging 12.3off-flavor days per pond). The number of off-flavordays in the post-September 18 period, estimatedbased upon laboratory research on off-flavor purg-ing rates, was also significantly different betweentreated and untreated ponds.

When the additional production costs for de-layed harvest and copper sulfate applications were

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TABLE 6.—Total off-flavor costs (US$) calculated for each sampling period in 2002 treatment and individual pond.Letters represent significant differences (P , 0.10) for values within a column.

Pond numberor variable

Surface area(hectares)

16 Jul–27 Aug

28 Aug–18 Sep After 18 Sepa Total

Control

9N 4.5 0 865 0 86512N 4.5 0 865 157 1,02313N 4.5 1,652 865 157 2,67519N 6.5 0 0 0 020N 6.5 0 0 229 22923N 6.5 2,403 1,259 229 3,891

113 5.3 0 930 186 1,116114 2.6 1,352 0 0 1,352115 3.2 1,202 0 86 1,287160 4.1 1,502 715 0 2,217162 4.1 0 0 0 0164 4.5 1,652 787 157 2,596All 57.8 z 9,763 6,287 1,202 17,252Mean/pond 814 z 524 z 100 z 1,438 zMean/hectare 169 109 21 299

Treated

5S 4.1 172 57 0 2307S 4.1 172 57 0 230

16N 4.5 1,842 929 157 2,9282W 4.1 1,674 57 0 1,7323W 4.1 172 57 0 2304W 5.3 224 75 0 298

112 5.3 224 75 0 298116 3.2 138 618 114 870117 3.2 138 46 0 184161 4.9 207 69 0 275163 3.6 1,507 695 0 2,202165 4.5 1,842 63 0 1,905All 50.9 z 8,311 2,799 272 11,381Mean/pond 693 z 233 z 23 y 948 zMean/hectare 163 55 52 224

a Off-flavor days for the post-18 Sep period were enumerated based on research (Johnsen et al.1996; Howgate 2003) on depuration times required to get catfish on-flavor for various levels of2-methylisoborneol.

calculated for each pond within a treatment, therewas no significant difference (P . 0.10) betweenthe treated and control ponds for any of the sam-pling periods (Table 6). Even though overall costswere greater for control ponds ($1,438 per pond)than for treated ponds ($948), the difference wasnot statistically significant because of the high var-iation among ponds.

Tucker et al. (2001) found that copper sulfateapplication to experimental ponds yielded a sig-nificant economic benefit, whereas our presentstudy did not. In the study conducted by Tuckeret al. (2001), economic analyses were based onresults from 18 carefully managed experimentalponds that were sampled over 3 years. In the pres-ent study, economic analyses were based on 24commercial ponds that were sampled for off-fla-vored fish over one summer. However, ecologicaldata collected over the 3-year duration of the pres-ent study show that abundance of odor-producing

cyanobacteria, levels of MIB, and prevalence ofoff-flavors in fish were reduced by copper sulfatetreatment. Therefore, we believe that our inabilityto demonstrate economic benefits did not resultfrom treatment ineffectiveness but rather from thesmall data set used for economic analyses. Thelimited data set, coupled with the large temporaland among-pond variation in water quality that istypical of aquaculture ponds (Boyd and Tucker1998), precluded a statistically significant resulteven though copper sulfate treatment reduced po-tential harvest delays by nearly half and reducedcosts associated with off-flavor by 35%. We feelconfident that a more extensive study would haverevealed significant economic benefits.

Conclusions

The results from this study indicate that weeklylow-dose applications of copper sulfate reducemusty off-flavor problems in farm-raised channel

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146 SCHRADER ET AL.

catfish in commercial-size ponds. The rapid dis-persal of the copper sulfate by use of aerators helpsto expose phytoplankton to adequate phytotoxicconcentrations of copper before the element dis-appears from the water. Treatment will cause de-terioration in water quality, especially higher totalammonia and nitrite concentrations. However, oneof the benefits of the low-dose protocol we usedis that deterioration of water quality is not as se-vere as might occur through the use of higher cop-per concentrations.

There are several management recommenda-tions that channel catfish producers must considerto help with the effectiveness of this type of treat-ment. The burlap bags containing copper sulfateshould be suspended on the side of the aeratorsopposite the pond bank if possible. This placementof the bags will help in the quick and even dis-tribution of copper throughout the pond water.Paddle-wheel aerators were used in this study, andwe recommend use of the same type of aeratorsince the effectiveness of other aerator types (e.g.,pump sprayer) for copper distribution is unknown.The size and shape of the pond must be consideredin determining the proper number and placementof aerators to achieve full and even distribution ofthe copper throughout the pond. Most of the pondsreceiving copper sulfate in our study were 3–6 ha,which is the typical size range of most commercialponds used for catfish aquaculture. The actualpond volume must also be known so that the properamount of copper sulfate is used. As mentionedby Tucker et al. (2001), water quality must be con-sidered because high total alkalinity and pH willpromote the rapid loss of copper from the pondwater. On the other hand, waters with low totalalkalinity and pH will increase the copper’s tox-icity to fish (Boyd and Tucker 1998).

Weekly low-dose applications of copper sulfateshould be used in ponds that are scheduled forharvest during the summer or early autumn, es-pecially those that have had a chronic history ofmusty off-flavor episodes. Applications shouldcommence in the spring when the pond water tem-perature rises above 208C, since this is the tem-perature above which O. perornata becomes moreprevalent in catfish ponds. Delays in applicationmay permit an increased prevalence of O. peror-nata in the ponds, thereby leading to musty off-flavored channel catfish that require additionaltime to purge the musty taint. While most catfishfarmers currently use diuron to mitigate musty off-flavor, the results of our study indicate that weeklylow-dose application of copper sulfate at com-

mercial-size catfish operations is an alternativemanagement approach that is beneficial in reduc-ing MIB-related off-flavor in pond-cultured chan-nel catfish.

Acknowledgments

Reed Doyle and Rick Fernandez, farm managersat the aquaculture facilities where this study wasconducted, are greatly appreciated for their time,willingness, and efforts to ensure proper applica-tions of copper sulfate were made and for provid-ing farm management data and other pertinent in-formation related to the study. We also thank mem-bers of the sensory panel, including Mayme Pick-ens and Tim Santucci. The technical assistance ofMargaret Dennis, Ramona Pace, Dewayne Harries,Yarda Tables, and Misty Schaubhut is greatly ap-preciated. Reference to trade names does not implyendorsement by the U.S. Government.

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